(1) Field of the Invention
The invention relates to a method for determining flow conditions in a measuring volume, which is flowed through by a fluid seeded with optically detectable particles, comprising the following steps:
a) Simultaneous recording at a first recording point in time of a first plurality of two-dimensional, real images of a three-dimensional, real distribution of the particles by means of the same plurality of image detectors arranged spatially offset with respect to one another,b) Repetition of Step a, at least at one second recording point in time,c) For each recording point in time: determination of a three-dimensional, estimated distribution of the particles based on the real images andd) Calculation of a three-dimensional displacement vector field by comparing the estimated distributions.
(2) Description of Related Art
Methods of this type are known as 3D-PTV, Three-Dimensional Particle Tracking Velocimetry. Details on this are known, for example, from Th. Dracos (ed.), Three Dimensional Velocity and Vorticity Measuring and Image Analysis Techniques, 209-227 (1996) Kluver Academic Publishers. 3D-PTV is based on the basic concept of individually determining the individual positions of a plurality of particles in a measuring volume at different points in time by means of triangulation. By comparing the distributions determined at these different points in time, a displacement vector can be determined for each individual particle, which represents the movement of the particle between the two measuring points in time. Specifically, a measuring volume is simultaneously recorded by several cameras, typically by three or four cameras, at different observation angles. In each of the 2-dimensional images recorded, the 2-dimensional particle position is determined for each imaged particle. Subsequently, the 3-dimensional particle position in the measuring volume is determined from the review of the different images, with the knowledge of the respective observation angle for each imaged particle. This step is generally referred to as triangulation. This method is carried out at least two successive points in time, so that at least some of the particles in the measuring volume change their position slightly but in a clearly measurable manner between the recording points in time due to the flow in the measuring volume. In a step often referred to as tracking, the corresponding particle positions determined at the different points in time are assigned to one another. A comparison of the particle positions assigned to one another leads to the calculation of a displacement vector field, which contains a vector for each particle in the measuring volume or for each volume element of defined size, which vector represents the respective flow-induced displacement between the observation points in time. A disadvantage of this method lies in its limitation to comparatively small particle numbers and particle densities in the measuring volume. A particle density that is too large leads in the imaging of the three-dimensional measuring volume to multiple overlapping of the images of the particles in the camera images recorded, so that the determination of the 2-dimensional particle positions is inaccurate and faulty. In the triangulation, major errors thus occur in the determination of the 3-dimensional particle positions. For example, existing particles are not found or non-existent particles, so-called “ghost particles”, are “found”.
This can be remedied in part by the use of a larger number of detectors in different spatial positions, which view the measuring volume at different observation angles. However, this approach can be pursued only to a limited extent due to reasons of space and cost. Furthermore, with the growing number of detectors, i.e., with a growing number of images to be taken into consideration in the triangulation, the computing time and thus the measuring time are increased substantially. However, the speed resulting from its mathematical simplicity is precisely one of the particularly advantageous properties of 3D-PTV, which in general one wants to retain.
If the time aspect is not important, other known methods can be used, in particular PIV, Particle Imaging Velocimetry, with tomographic reconstruction, known as Tomo-PIV for short. In this method, which is known from EP 1 517 150 B1, likewise two-dimensional images of a three-dimensional measuring volume are recorded at different observation angles and in pairs at different points in time. The measuring volume in Tomo-PIV can be seeded with a much higher particle density compared to PTV. However, no individualized observation is carried out of individual particles, the position of which is determined by triangulation. Instead, tomographic reconstruction methods are used, which subdivide the measuring volume into a multiplicity of fixed voxels and calculate mere intensity distributions. A three-dimensional displacement vector field is then determined by cross correlation techniques, which are applied to the voxel-based, three-dimensional intensity distributions. The advantage of Tomo-PIV lies in the high density of the displacement vectors achievable in the resulting three-dimensional displacement vector field. However, the method is very computationally intensive and therefore time-consuming.